ARM: dts: stm32: Fix touchscreen node on dhcom-pdk2

Fix make dtbs_check warning:
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml:0:0: /soc/i2c@40015000/polytouch@38: failed to match any schema with compatible: ['edt,edt-ft5x06']

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>